Electrically detected and conventional magnetic resonance investigation of surface
and bulk states in polyaniline thin films
Fernando A. Castro and Carlos F. O. Graeff
Citation: Journal of Applied Physics 101, 083903 (2007); doi: 10.1063/1.2719007
View online: http://dx.doi.org/10.1063/1.2719007
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/101/8?ver=pdfcov
Published by the AIP Publishing
Electrically detected and conventional magnetic resonance investigation
of surface and bulk states in polyaniline thin films
Fernando A. Castro
Departamento de Física e Matemática, FFCLRP, Universidade de São Paulo, Av. Bandeirantes 3900, 14040-901 Ribeirão Preto, SP, Brazil
Carlos F. O. Graeff
Departamento de Física, FC, UNESP, Av. Luiz Edmundo Carrijo Coube, 14-01, 17033-360 Bauru–SP, Brazil
共Received 31 October 2006; accepted 21 February 2007; published online 25 April 2007兲
Electrically detected magnetic resonance共EDMR兲and electron paramagnetic resonance共EPR兲were used to investigate emeraldine base polyaniline films. The magnetic susceptibility presented a Curie
共localized spins兲—Pauli共delocalized spins兲transition at 240 K, when we also observed a transition in the dependence of the g factor with temperature 共T兲. Peak-to-peak linewidth decreases with increasing temperature, reflecting that motional narrowing limits the hyperfine and dipolar broadening in this polymer. EDMR spectra could only be observed above 250 K in accordance to EPR results. Surface and bulk transport could be separated and their analysis reflected the effect of magnetic interaction with oxygen. ©2007 American Institute of Physics.
关DOI:10.1063/1.2719007兴
I. INTRODUCTION
Conducting polymers have attracted increasing interest over the last decade due to the particular properties of the electronic states in a one-dimensional structure, as well as the near metallic conductivity that can be achieved upon doping.1 Polyaniline is one of the most studied conducting polymers due to its higher stability in air and its potential for commercial applications in various fields, such as electron-ics, electromagnetic shielding and sensors, among others. Its conductivity can be changed by protonation of the insulating emeraldine base form of polyaniline共EB-PANI兲.
Although all spins are paired in an ideal EB-PANI, even carefully synthesized samples yield an electron paramagnetic resonance共EPR兲signal. The nature of these spins is still the subject of debate.2Investigation of the magnetic susceptibil-ity共兲as a function of temperature共T兲often yields a Pauli-type behavior共 independent of T兲, even for relatively low dc conducting polymers.3However, in some cases, a transi-tion from Curie to Pauli-type temperature dependence has been observed.3–5Curie susceptibility is a characteristic sig-nature of localized spins, while Pauli susceptibility is char-acteristic of delocalized spins. Why this transition is not al-ways observed and how it relates to transport properties is still not completely clear.
Much progress has been made in the understanding of the conduction mechanism in different polymers; however, a detailed microscopic picture of the transport processes is not yet established for many of them. Such an investigation is an intricate subject. First, because of the very complex structure and morphology of conducting polymer films. Second, be-cause only a few experimental techniques are capable of di-rectly probing these processes selectively on the microscopic scale.
Electrically detected magnetic resonance共EDMR兲 mea-sures the change in the sample conductivity under magnetic
resonance condition. Thus it can relate microscopic proper-ties to charge transport mechanisms, for example, identifying the electronic states taking part in the process. In fact, over the past years it has provided valuable insight into various transport and recombination processes in semiconductors.6–11 In this work we have used EDMR and EPR to investi-gate emeraldine base polyaniline films. We observed a tran-sition of the magnetic susceptibility dependence on tempera-ture, from a linear decrease with increasing temperature
共localized Curie spins兲to a constant value共delocalized Pauli spins兲. The g factor also showed a transition at the same temperature 共240 K兲. EDMR spectra could not be observed below 250 K in agreement with EPR results. The investiga-tion of both surface and bulk transport yielded different spec-tral features that were explained by the influence of oxygen and water molecules.
II. EXPERIMENT
Deprotonated EB-PANI was obtained commercially. Films were prepared by casting a 0.1 M N-methyl-2-pyrrolidone共NMP兲solution on glass and heating it at 60 ° C until full evaporation of the solvent. At room temperature, films were immersed in milli-Q water and floated off the substrate. After drying we obtained flexible, free-standing films with typically a 20m thickness. For EDMR, samples were cut to approximately 4⫻8 mm2, and either coplanar
共0.5 mm spacing兲orsandwichsilver contacts were deposited
共with a contact area of 0.2 cm2兲.
EPR and EDMR measurements were done using a modi-fied, computer interfaced, Varian E-4 X-band spectrometer. The temperature was varied using a LN2flow cryostat, typi-cally between 168 and 300 K. At each step, we allowed the temperature to stabilize for at least 30 min. The conductivity spin-dependent changes were measured by modulating the static magnetic field 共H0兲 and using lock-in detection of the
0021-8979/2007/101共8兲/083903/5/$23.00 101, 083903-1 © 2007 American Institute of Physics
resonant current changes. Magnetic susceptibility data were obtained from EPR spectra as being proportional to the sig-nal intensity 共the area under the absorption curve兲. DPPH was used as the standard to calibrate the measurements of the
g factor.
III. RESULTS
A. Electron paramagnetic resonance
Figure 1共a兲 shows the EPR signal of an undoped EB-PANI film at 300 K. The peak-to-peak linewidth共⌬Hpp兲
is 2.6 G and the g factor is 2.0031. These values are in agreement with those found in the literature, although the reported values may vary from40.29 to 23 G.12These differ-ences are due to magnetic interaction of polarons and the triplet state of oxygen molecules,13which broadens the reso-nance line.
Fitting the EPR signals, we observed that it is composed of two Lorentzian lines: a narrow one, about ⌬Hpp= 2.3 G,
and a large one, about⌬Hpp= 17− 23 G, depending on
tem-perature关Fig.1共b兲兴. It should be noted that it is not possible to simulate our signals using only one Lorentzian line.
At all measured temperatures, from 168 to 300 K, the EPR spectra were always composed of two lines with more or less equal intensities. Polyaniline EPR spectra composed of two lines have already been reported.14,15Mizoguchiet al.
observed a single narrow Lorentzian for undoped samples, but as they raised the doping level, a second Lorentzian started to appear. Under very high doping levels this second component can become dominant. The appearance of this second component was also observed in an iodine doped polymer from the PPV family.3 In that work, the magnetic susceptibility dependence on temperature is very similar to that observed in our polyaniline samples, as will be dis-cussed further in this paper.
The temperature dependence of the EPR signal is domi-nated by the narrow component. Therefore, the line shape analysis will be carried out only for this component. If we observe the temperature dependence of the magnetic suscep-tibility共兲, which is proportional to the area under the inte-grated EPR spectra, shown in Fig. 2, we can clearly see a transition behavior at approximately 240 K.
The solid line, used to guide the eyes, shows that de-pends on the inverse of the temperature共1 /T兲 between 168
and 233 K, following Curie’s Law. Then, there is a sudden increase and the value of becomes temperature indepen-dent between 258 and 300 K, showing a Pauli-type behavior. Note that between 233 and 248 K the susceptibility depen-dence on temperature deviates from 1 /T. The possible reason for this behavior will be discussed further on. Such a Curie-Pauli transition has already been observed in polyaniline4 and in other conducting polymers,3,5 such as polypirrol and polythiophene. However, the temperature共Tc兲, at which this
transition occurs, varies as a function of the sample’s doping and crystallinity level.16,17
The peculiar behavior of the temperature dependence of the dominant’s line g factor, shown in Fig.3, confirms that an important change occurs at this critical temperature. Theg
factor value decreases from 2.0031 to 2.0028 as we raise the temperature from 168 to 228 K. Then, there is a sudden increase to 2.0031 andgremains practically temperature in-dependent between 233 and 300 K. The reason for this be-havior is not completely understood at the moment; however, there is a clear relation to the Curie-Pauli transition.
The temperature dependence of the dominant compo-nent’s linewidth is shown in Fig.4共a兲. The solid line is used to guide the eyes. We observe that ⌬Hpp decreases as
tem-FIG. 1. 共Color online兲 共a兲EPR signal of a polyaniline film at room tempera-ture;共b兲Integrated EPR signal共solid spheres兲decomposed into two Lorent-zian lines, a narrow共dashed line兲and a broad 共dotted line兲 one. The solid line represents the sum of the two components共L1 + L2兲.
FIG. 2. Magnetic susceptibility temperature dependence for a PANI thin film.
083903-2 F. A. Castro and C. F. O. Graeff J. Appl. Phys.101, 083903共2007兲
perature is increased. In general, EPR linewidths are deter-mined by the interplay of spin-spin interactions 共hyperfine, dipolar, etc.兲, spin-lattice relaxation and narrowing mecha-nisms共spin diffusion, rotation, and exchange兲.
In the case of conducting polymers, the hyperfine and dipolar broadening of a resonance will be limited by narrow-ing due to spin diffusion.18 An increase in conductivity, as observed in Fig.4共b兲, will increase the effectiveness of spin diffusion to narrow the EPR linewidth.
B. Electrically detected magnetic resonance
No EDMR signal could be observed at temperatures be-low 250 K. Different from the EPR results, the EDMR signal from polyaniline devices is dominated by a broad line. The signal isenhancing, meaning that conductivity increases dur-ing resonance, as would be expected for a hoppdur-ing conduction.6,11Two processes could result in this EDMR sig-nal: interchain or intrachain polarons hopping. Note that in order to obtain an EDMR signal it is necessary that two spins participate in the observed process. One polaron will contrib-ute to the signal when it hops to a site already occupied by another polaron. The resonance condition will enhance con-ductivity because it increases the spin dependent transition
共hopping兲rate.
Figure 5共a兲 shows the integrated EDMR signal 共open circles兲 of a coplanar device, at 294 K and 50 V共electric field of the order of 103V / cm兲. The signal is also composed of two lines: a broad and a narrow line. The solid line rep-resents the sum of the two components.
The narrow component has a g factor of 2.0033 and
⌬Hpp= 3.2 G. This value is very close to that of the narrow component of the EPR signal. The broad component, on the other hand, has a g factor slightly different, of 2.0035 and
⌬Hpp= 14.5 G. Graeff et al.
7
have observed a strong depen-dence of the narrow line intensity with the applied voltage. According to the authors, the narrow component is dominant at low electric fields共⬍103V / cm兲and strongly decreases as FIG. 3. Dependence of the thin componentgfactor on temperature.
FIG. 4. Temperature dependence of:共a兲the peak-to-peak linewidth of the dominant EPR component;共b兲EB-PANI conductivity at 104V / cm.
FIG. 5.共Color online兲Integrated EDMR signal of:共a兲A coplanar contacted device共open circles兲, decomposed into two components: a broad Gaussian-type共dotted line兲and a narrow Lorentzian-type共dashed line兲. The solid line represents the sum of the two components共L1 + L2兲;共b兲asandwichtype device共open circles兲fitted using one broad Gaussian line共solid line兲.
the field is increased. The appearance of a broad component was only observed for fields above 103V / cm.
The EDMR signal fromsandwich devices is composed of only one broad line with g factor of 2.0035 and
⌬Hpp= 22 G. The integrated signal共open circles兲and the
fit-ted line共solid line兲can be seen in Fig.5共b兲. Possible reasons for the differences observed between coplanar andsandwich
devices will be discussed further on.
IV. DISCUSSION
EPR measurements on polyaniline films have shown a transition in the spin system from Curie-type to Pauli-type, as we raise the temperature. Below ⬃240 K, magnetic sus-ceptibility decreases with increasing temperature and above this temperature it is independent ofT. Curie spins are local-ized, therefore, they do not contribute to the current, and, as a consequence, to the EDMR signal. On the other hand, the Pauli-type contribution comes from delocalized spins, which do contribute to the current.19 That could explain why no EDMR signal is observed below 250 K for these polyaniline samples.
Numerical calculations17,20 have shown that at suffi-ciently low temperatures, so thatkBT, wherekBis Boltzmann
constant, is smaller than the average Coulomb interaction energy 共U兲 among electrons, states near the Fermi energy
共EF兲will become singly occupied, and magnetic
susceptibil-ity will exhibit a Curie behavior
共T兲=B2N S/kT,
whereNSis the number of singly occupied states per volume
unit. Considering a low density of states at the Fermi energy, the Curie contribution at low temperatures comes from the occupancy of localized states close toEF.
WhenEFis close to the mobility edge and much higher
than the interaction energyU, the spin susceptibility depen-dence on temperature will gradually change from a Curie-type to a Pauli-Curie-type contribution, as we raise T. The transi-tion region occurs whenU⬇thermal energy.
Since crystallinity and doping levels can alter the Fermi energy and/or electron-electron interaction energy, different research groups have 共or have not兲 observed the transition present in Fig.2depending on doping and/or the crystallinity level of the studied samples.4,16The transition temperature is higher where there is higher disorder and/or at lower doping levels. As most of the EPR studies on polyaniline are carried out with the polymer in powder, many research groups only observed the Curie-Pauli transitions in highly doped samples.14 In those cases, they observed that the signal was composed of two Lorentzian lines, a broad and a narrow one, in agreement with our results. However, in Ref.14the broad component was dominant, while in our case it is not. Accord-ing to Mizoguchi et al.,14 the broader component is not present in EPR spectra of undoped polyaniline samples, but starts to appear as the doping level is raised, becoming domi-nant at high doping levels. One interesting question is the origin of such behavior.
It is believed that H2O molecules are adsorbed preferen-tially by localized spin sites in polyaniline.12 Since the
ad-sorption sites, where the polaron is centered 共i.e., NH+兲, are the same for O2 and H2O molecules, the higher water ad-sorption leads to less free sites for oxygen to be adsorbed. Houzé and Nechtschein13 proposed that H2O molecules are more tightly bound to the polymeric polyaniline chain than O2 molecules, and that water acts as a shield between po-larons and oxygen. Thus, as the doping level is increased, we have more delocalized polarons, and, consequently, more sites for oxygen adsorption, which will broaden the EPR signal due to dipole-dipole interactions with paramagnetic species. The fact that the narrow component is dominant in our spectra comes from the low共unintentional兲doping level of our samples.
Peak-to-peak linewidths 共⌬Hpp兲, equal to
12
2.2 and 2.0 G,13have been reported, respectively, to samples in the water vapor atmosphere and ambient atmosphere. These results are very close to the values of ⌬Hpp obtained in this work
共⌬Hpp= 2.6 G兲, in which the films were prepared and kept in
air. The fact that we observe two components, a narrow and a broad one, represents the contribution of both localized
共sites mostly occupied by H2O兲and delocalized共sites mostly occupied by O2兲polarons.
21
In EDMR measurements, the broad signal is dominant since it is due, mostly, to delocalized polarons. However, at low bias, for coplanar contacted devices, we observe only a narrow component of decreasing intensity as bias is raised.7 The narrow component, in analogy to EPR,12,13is attributed to polarons near H2O, meaning, in this case, polarons close to the surface. Near the surface, water molecules have a higher probability of being adsorbed, lowering the effect of oxygen broadening. As bias is raised, the electric field starts to further penetrate into the bulk, where there is no longer the competition between H2O and O2molecules. At a higher bias, the bulk component increases relative to the surface component, and the intensity of the narrow line decreases. This same idea can be applied to EPR. The samples in pow-der form have a much higher surface area relative to films. Thus, as observed,14EPR signals in this case are dominated by one narrow Lorentzian line.
Curie-Pauli transition
Mizoguchi16and Wang et al.19 have shown that macro-scopic conductivity is dominated by interchain and/or inter-metallic island hopping. According to Krinichnyi,22 macro-scopic conductivity in polyaniline, both ac and dc, is thermally activated at approximately 200 K. Therefore, we have attributed our EPR signals, at high temperatures, to movable polarons thermally activated above approximately 220 K. The EDMR signal is attributed to interchain hopping. The formation of bipolaron states is energetically favorable relative to the polaronic formation.3 Thus bipolaron forma-tion increases with decreasing temperature, leading to smaller magnetic susceptibility values. When the polaron density is small enough, the fusion 共bipolaron formation兲
process no longer takes place and the Curie behavior at low temperatures is observed due to the fixed number of local-ized polarons. This also explains why the susceptibility de-viates fromT−1dependence, between 233 and 248 K.
083903-4 F. A. Castro and C. F. O. Graeff J. Appl. Phys.101, 083903共2007兲
V. CONCLUSION
We used electron paramagnetic resonance and electri-cally detected magnetic resonance to investigate polyaniline films. We observed a Curie-Pauli spin transition as a function of temperature that is reflected in an abrupt change of theg
factor. The EPR linewidth decreased linearly with an increas-ing temperature due to an enhanced spin diffusion. The study of coplanar and sandwich type devices highlighted different EDMR signals for surface and bulk currents. We concluded that EPR signals are also composed of a surface and a bulk contribution. The competition between oxygen and water ad-sorption on the surface could account for the differences ob-served. Due to the strong influence of oxygen on the line-width, we propose that EDMR can access encapsulations. Any oxygen induced broadening would be readily detected and, different from EPR, no signal from the materials used for encapsulation would be observed.
ACKNOWLEDGMENTS
The authors are pleased to acknowledge financial sup-port from the following Brazilian agencies: FAPESP and CNPq共IMMP兲.
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